U.S. Pat. No. 4,150,711 is entitled "Method and Apparatus for Continuously Casting Metal Slab, Strip or Bar with Partial Thickness Integral Lugs Projecting Therefrom" (R. W. Hazelett and J. F. B. Wood). It is assigned to the same assignee as the present application and is incorporated herein by reference. The continuous casting machine described therein employs two revolving edge dam strings, the blocks of which are strung in closed loops disposed opposite each other in parallel, the blocks being strung upon longitudinal high-strength unifying members there shown as straps. The edge dam blocks include special blocks having partial thickness mold pockets for forming integral, cast-in protruding shoulders or lugs on the cast product. In one existing installation, there are nine such special dam blocks uniformly spaced around each edge-dam loop. By the term "block" in reference to the elements of edge dams, we mean to include not only rectangular shapes but other shapes such as the special blocks just mentioned and trapezoidal shapes.
The cast product issuing from the continuous casting machine may be cut by means of a shear or torch into anodes for the electrolytic refining of copper. Presently, for example, each anode is on the order of 36 inches (900 mm) long. The pair of edge dams is synchronized and controlled through a feedback loop to maintain the profile or shape of the product being continuously cast. The cast-in protruding shoulders or lugs serve as hangers or supports for each anode and provide electrical connections for the anode as it hangs by its shoulders in the electrolytic tank. The anode thus formed may be so profiled as to minimize scrap. The protruding shapes so cast may be the full thickness of the cast product generally, or they may be some fraction of the thickness, or some combination of the two. But for the use of such projections or lugs in the casting of anodes, the lugs should normally be in directly opposed, square, accurate alignment on the opposed parallel sides of the cast product. In this way, the resultant anodes can be closely and accurately suspended by means of these cast-in hangers or supports in the electrolytic solution of a tankhouse without touching the sides of the tank. This control of alignment can be done in such a uniform way as to avoid electrical arching of short circuits commonly caused by imprecise alignment and contact between anodes. In order for the lugs or integral supporting shoulders to be formed in the usually desired mutually square alignment, or in any other consistent alignment, synchronization of the movements of the edge dams traveling along the opposed edges of a moving mold must be controlled and maintained. Without such synchronization control, random factors such as temperature variations will cause initially synchronized edge dams to drift or diverge in their alignment relative to each other; one edge dam may then be said to "walk away" from the other.
The same requirements and considerations are applicable when the edge dams are utilized for a different purpose, that is, not to cast spaced lugs, but for example to cast spaced lobes or recessed hollows for purposes hereinafter described.
The need for edge-dam synchronization arises from perhaps five sources of cumulating offset or error in the relative positions of the mold pockets in the two moving edge dams which serve to cast the lugs. Three of these error sources involve the effects of varying amounts of heat transfer at the edges of the moving mold, causing differential thermal expansion of the respective edge dams, and the other two involve the effective lengths of the blocks and their end-to-end spacing:
First, one edge dam may heat up more than the other because of the flow of the incoming molten metal being inadvertently directed more toward one than the other entering edge dam.
Second, unpredictable variation arises mainly from the effects of interstitial air gaps or interstices of varying thickness occuring between the product being cast and the respective blocks of the edge dams, across which gaps heat from the solidifying molten metal is extracted. Such air gaps result in part from varying local shrinkages of the product, or variations in pressure exerted upon the edge dams by mechanical constraints--for example, by the side guides external to the dams as well as the belts, all of which extract heat from the dams. Air gaps may also result from imperfect squareness or wear of the downstream in-line pinch roll stand, which engages the cast product soon after issuing from the moving mold (downstream being defined as the direction of metal product flow). Such out-of-squareness about a vertical axis will drag the emerging product slightly sideways, thus pressing it harder against the dam blocks of one edge dam. The pinch rolls grip the frozen product soon after it emerges from the casting machine and mechanically insulate the operation of the casting machine from the effects of subsequent equipment such as a rolling mill or cutoff shear. The pinch rolls are carefully synchronized with the casting machine though they are normally driven at a slightly slower, carefully adjusted speed, in order to maintain the cast product in a state of compression until it is cooled below the range of hot-shortness.
A third source of variable heat transfer, likewise unpredictable, arises from films of oxide, etc., of varying thicknesses upon both the edge dams and on the nearby regions of the product, again causing variations in heat flow from the metal being cast into the dam blocks and consequently causing variations in the heating of these blocks.
A fourth general source of cumulative error can arise from variations in the original, as-manufactured length of the dam blocks, in the longitudinal direction of their movement. Tolerances in the length of each of the many individual dam blocks cumulate into significant error in the length of a whole dam loop. Normally, a tolerance of around .+-.0.001 inch (25 micrometers) is maintained in the manufacture of the blocks. However, the last block to be assembled into an edge-dam loop can be specially machined to a length that compensates for cumulative error of length in block manufacture.
Fifth, foreign matter such as oxide scale from heat, or mineral deposits from evaporated cooling water, may find its way between the dam blocks, thereby changing their end-to-end spacing or effective lengths.
The foregoing factors combine to render powerful means of synchronization fully necessary--preferably automatic in order to achieve high levels of uninterruped production of high quality cast product.
Synchronization is achieved in the method and apparatus described in the aforesaid patent by sensing the relative errors in position of the laterally extending depressions in the traveling edge dams or by sensing the cast lugs after they exit from the casting machine. The temperatures of the revolving edge dams are then controllably changed with respect to each other. This change in relative temperature is accomplished by increasing the heating or cooling of at least one of the pair of revolving edge dams when it tends to lag the other, thereby altering its loop length and thus altering its rate of completing its revolutions. If an edge dam is cooled and thereby shortened, it completes its revolutions faster and so tends to gain in position. If an edge dam is heated, the opposite occurs.
U.S. Pat. No. 4,586,559 for "Process and Apparatus for Casting a Strip with Laterally Extending Lugs" (M. K. Govaerts, H. A. L. Gielen and J. M. A. Dompas) is assigned to the same assignee as the present application and is incorporated herein by reference. In that patent application, strings of edge-dam blocks are described as being deliberately heated as by means of radiant gas burners, or cooled by means of aqueous sprays, with the object of synchronizing the progress or cycling of the edge dams. Both heating and cooling may be employed in conjunction with each other in order to enhance the effectiveness of synchronization of the traveling edge dams.
The heating and cooling may be carried on automatically in a "closed-loop" control in order to free an operator from the task of manual alignment control, thereby enabling the use of the operator's time elsewhere on the continuous casting line. However, it has been found that heating and/or cooling within acceptable limits of temperature may not always be powerful enough to maintain synchronization of the traveling edge dams, i.e., not powerful enough to prevent the desired "closed-loop" mode of control from inadvertently turning into an unstable "open-loop" whereby one edge dam progressively "walks away" from its desired location with respect to the other in a situation of progressively increasing error between the lug positions on opposite edges of the cast product. The result is the casting of irregularly formed anodes which cannot be suspended in the electrolytic tanks of the tank-house.
During any kind of continuous casting, whether of anodes or of other products, there have at times occurred sizeable open gaps between successive edge-dam blocks. These gaps or spacings result from contingencies in the manufacture, use, and wear of the assembled edge dams. These open gaps between edge-dam blocks must be closed in order to complete the mold; i.e., molten metal must be prevented from flowing or "flashing" and wedging into spaces between successive dam blocks and then freezing into troublesome fins which are apt to cause interruptions in casting. In extreme cases, fins can cause leaks which are especially dangerous in the casting of steel, since steel with its normally molten oxides displays a low surface tension--rather like that of water.
U.S. Pat. Nos. 3,865,176 and 3,955,615 are assigned to the same assignee as the present application and are incorporated herein by reference. As therein described, gaps occurring between successive edge-dam blocks are closed into a relation of mutual closeness in the mold by causing each edge dam, while in use, to follow a path such that its own geometry causes many of the edge dam blocks to press endwise together while strung upon the high strength longitudinal unifying member shown as a metal strap. More specifically, each edge dam in its return path back to the moving mold is made to pass, after exit from the mold, over a roller tensing apparatus with the rollers disposed so as to conform approximately to a smooth curve. The roller tensing apparatus are commonly known as "back-breakers." Each is usually assembled and configured into a system with a movable cooling chamber which incorporates a cooling apparatus, together with sensors and automatic controls and actuators. Back-breakers are so mounted so as to be controllably forced or lifted up against the edge dam loop. When moved upwardly, the back breaker causes the edge dam so deflected to travel locally along a smoothly curved arc which is concave as seen from the exterior of the edge dam loop. Any slack or open gaps between the dam blocks in the moving mold are taken up by the geometrical effect of this local concavity along the return path of the edge dam. Specifically, the geometrical effect is that of causing wedge-shaped spaces to occur between the dam blocks along the arc of this local concavity in such a manner as to cause such block effectively to occupy more space along its strap (see FIG. 3 of U.S. Pat. No. 3,865,176), thereby compacting or pressing together the remainder of the blocks in the edge-dam as a whole along each whole string or loop, which string or loop stays about the same length. In this way, "flashing" of molten metal into gaps between successive blocks is effectively prevented.
In summary, up until the present invention, the only known function for the back-breaker apparatus, in its deflecting a local arcuate concavity in each return reach of each edge dam loop, was to cause the remainder of the dam blocks to be pressed endwise together in order to avoid gaps between successive blocks, thereby avoiding the troublesome "flashing" of molten metal into such gaps, which flashing produces undesired fins on the cast product and which fins or flashings occasionally disrupt the continuous casting operation. In other words, for many years in many different countries, the technical experts in many different companies have been using twin-belt continuous casting machines with back-breakers for closing up the gaps between the edge dam blocks for preventing flashing of molten metal into gaps between blocks, and no one has previously conceived of relatively elevating and lowering the respective back-breakers for synchronizing the movements of the edge dams.